For the past four months, a single weather pattern has gripped virtually the entire United States, causing a coast-to-coast drought unique in the annals of weathers recording.

Water emergencies have been declared all along the East Coast; farmers from Colorado to Pennsylvania are seeing damage to winter crops and are fearful about their spring ones; ski resorts, despite some snow in the past few days, have reported disastrously low levels, even bare ground in several areas that usually have 60 or 70 inches.

The drought's most visible effects are in the Midwest. The Mississippi River, which draws water from 40 percent of the United States -- from the Rockies to the Appalachians and from Canada to the Gulf -- dropped off the water gauge at Memphis, in early January.

Officials installed additional gauges, but by the 10th of the month, the river had sunk below the lowest water level ever recorded at the city. And it continued to drop until last week when it hit 24 feet below normal, the lowest since records began to be kept when Mark Twain used to pilot vessels past Memphis more than 110 years ago.

The low water has struck fear into the hearts of the river pilots. Scores of towboats and barges have run aground in the mud and sand between Illinois and Louisiana, closing the river to traffic at least six times since Christmas and for up to five days at a time.

"I've just come from a meeting with many of the captains of these boats and some of them have been on the river 25 years or more. They say they have never seen anything like it -- drought, yes, but never continuing for such a long time," said Les Sutton, president of Dravo-Mechling Barge Co. s

"If the drought continues, I can imagine things just gradually getting worse and worse, with river closures of up to a week or longer, and finally the [Army] Corps of Engineers will just not be able to maintain the channel at all," he said.

The Mississippi barges normally carry over half the grain, 20 percent of the crude oil, and 15 percent of the coal transported in the United States each year. In the past month, 126 towboats and 1,455 barges have run aground even though barge owners have reduced loads by 15 to 20 percent to lessen the draft of their vessels.

The economic losses to the barge transport industry are estimated in the millions of dollars, some of which is being passed through to food, chemical, oil, and manufacturing companies, which are suffering losses of their own in some cases because of the fouled river traffic.

The weather system causing the drought is one of the most unusual national patterns ever recorded. Many meteorologists feel it marks the end of a 40-year cycle, the end of the extremely good weather the nation has experienced since the Dust Bowl days. They believe that the United States is in for severe and highly erratic weather conditions like those that brought record snow, cold, and drought in three of the past four years.

"This is a very unusual, a unique weather pattern for the nation," said Roland Loffredo, chief hydrologist with the National Weather Service. "I've never seen anything like it and I don't think anyone else has either.

"Rain and snowfall is below normal in virtually every area of the United States -- I've never seen a pattern so broad. In the Midwest, the rainfall is up to 15 inches or more below normal," Loffredo said.

According to several meteorologists who have analyzed the situation, there are two reasons for the current weather pattern, both extremely unusual:

First, there are two very large and very persistent high-pressure systems that have planted themselves over the West Coast and Rocky Mountians and remained there for four months. In America such long-lived weather centers were almost unheard of until 1977, when a similar one blocked the west coast for months. Last summer, another pressed down the Midwest and part of the East, causing record high temperatures and continuing the water deficit that began the previous winter.

Second, the "jetstream" -- a bundle of powerful steering currents that move at speeds up to 300 miles per hour at 30,000 feet above the earth -- is not where it normally is. This is important because its high-speed wind current drive storm stystems before them as they meander across the continent.

Normally the jetstream would wander straight through the middle of the country from west to east, pushing storms into a position where they could pick up moisture from the Gulf of Mexico and, as they spin clockwise, deposit it on the Midwest and East.

When the jetstream alters course the meteorological results are dramatic.

In 1977, it skiped up and crossed Canada instead of the United States, dropping down south only when it reached the Great Lakes. This left the West in extreme dryness while the Midwest and East experienced severe Arctic cold mixed with snow created when the icy air met moisture moving northward from the Gulf of Mexico.

This year, because of the very long-lived high-pressure system sitting on the West Coast, the jetstream jumped again up into Canada. However, this time it has not turned south at the Great Lakes. It has continued across to the north and straight to the Atlantic Ocean.

So one great engine that drives the nation's storms is missing us almost completely.

One good dose of rain and snow did blast its way across the West last week, dumping up to nine inches of snow over parts of the Midwest over the weekend.

"That will be great for the ski resorts for about two weeks," said a spokesman for the National Weather Service in Denver. "But that is the only storm that has broken through the dry pattern dominating the nation's weather." By the middle of this week the Washington area should receive some rain from that system, but probably not enough to appreciably affect low river levels, ground water levels, or reservoir depletion.

"That one storm has passed through . . ., and we've gone right back and set up the very stable high pressure area that has caused the drought. It looks like we are going right back into the dry weather, and possibly for some time to come," the spokesman said.

And even a good dose of rain won't go far toward replenishing the Mississippi. "If it rains 24 hours a day from now unitl next week," said an Army Corps of Engineers spokesman, "that won't even bring it [the river] up to its old record [low]."

The fear of those who own boats and barges on the Mississippi, and the fear of the thousands of farmers whose lands lie in the Mississippi basin, is that this drought will not confine itself to winter in which it can do limited demage. They fear it will remain through February and even into March or April.

"Everything now depends on having heavier-than-normal rains in the late winter and srring," said Loffredo. "We are already at the bottom limits on our reservoirs and river levels. And we are still operating at a deficit from the dry conditions last summer, and even last winter. So we have to try to rebuild the water levels of three winters up.

"If we don't get those rains, the situation will get very serious. Very serious," he said.

The national Weather Service is now predicting rain below normal levels for all the United States in the next 30 days, except for a patch of the West including Texas, Arizona, New Mexico, and part of Colorado.

This prediction, if it turns out to be correct, means that the drought will continue through February, bringing the country toward major climatic disaster in the spring.

Some farmers have already reported that winter wheat appears stunted, and if the drought continues through the winter, a large percentage of their crops will be lost. To grow well in the spring, winter wheat must be planted in a moist soil, and then get a good snow cover to protect the sprouts from temperatures below zero degrees Fahrenheit, which can kill them. The current crop in most places had too little soil moisture to begin with, and now has little or no snow cover. Under these conditions the crop may be killed at temperatures even several degrees above zero.

In the Washington area, the Occoquan Reservoir has dropped from a 100-day supply of water to less than 40 days' worth. Another supply of fresh water for Washington residents, the Potomac River, has slumped to the second-lowest level in the 50 years that records have been kept. The Potomac is flowing at 1.3 billion gallons per day; normal flow is over 7 billion gallons.

If the drought continues, the river by next month could drop to 700 million gallons a day -- the danger level in terms of its ability to supply Washington with water.

The Chesapeake Bay had a record low inflow of fresh water in January, only about 20 percent of normal.

Normally, at this time of year, 55.3 billion gallons of fresh water flow into the bay every day; in January the average was 11.5 billion per day. This dribbling flow into the bay represents the rainfall and stream conditions over the entire 65,000-square-mile area of the Middle Atlantic states, and it was the lowest since records began to be kept 30 years ago.

As a result the salt front, where the ocean's salt water meets the flowing fresh water of the bay, has begun to move. No longer held back by the force of fresh water flowing from rivers into the bay, the salt front has now moved 28 miles inland.

Similar droughts conditions exist throughout the East, with record lows being set on streams in New Jersey, especially the Delaware River, which fell to 25 percent below the lowest level ever recorded. Reservoir levels that feed New York City have fallen to less than two-thirds of capacity.

Notices of fines totaling $4.2 million have been sent to thousands of businesses and residences in New Jersey in an effort to enforce the state's water rationing plan. The residents of wealthy Greenwich, Conn., have been asked to cut their water use by more than half, as that community is now left with less than an 18-day supply.